Twisted bourdon tube



Aug. 26, 1969 H. WEHDE ET AL TWISTED BOURDON TUBE Filed June 30. 1967 INVENTORS Heinz wehdea Werner Rles ATTORNEYS United States Patent Ofiice US. Cl. 73418 7 Claims ABSTRACT OF THE DISCLUSURE A twisted Bourdon tube made of a single piece of material and having a twisted central portion and two end portions, the two end portions having greater wall thicknesses than the central portion so as to be relatively rigid, and the twisted central portion having a star-shaped cross section.

Background of the invention The present invention relates to pressure transducers, and particularly to longitudinally twisted Bourdon tubes and methods for their manufacture.

It is known to fabricate twisted Bourdon tubes in the form of hollow bodies of noncircular, e.g., star-shaped, cross section, the bodies being permanently twisted, having a stationary longitudinal axis and being made of a resilient material. The ends of such tubes tend to rotate under the influence of an increased pressure in the interior of the tube, the direction of rotation being opposite the direction of the permanent twist. The use of twisted Bourdon tubes in pressure gauges is known, for example, from Control Engineers Handbook, John G. Truxal (Mc-Graw-Hill, Inc., New York) 1958, pages 17-72, and from Archiv fiir Technisches Messen [Archive for Technical Measuring] Dr. W. Wuest, January 1965, pages 14.

Although tubes of this type are in theory well-suited for particular measuring situations, they have certain shortcomings in that it is diflicult to fasten them at their ends, or to connect them with the indicator elements or measurement transducer members, for example. Deformations which have an unfavorable influence on the linearity of the measurement and which are also the cause of zero drifts, due to permanent deformations, occur articularly at these connective points.

The manufacture of such tubes is also diflicult in view of the requirement that sharp edges, which can later lead to cracks, must be avoided.

Summary of the invention It is a primary object of the present invention to overcome these drawbacks and difiiculties.

Another object of the present invention is to improve the stability of twisted Bourdon tubes.

A further object of the present invention is to manufacture twisted Bourdon tubes of uniform quality.

A still further object of the present invention is to provide a twisted Bourdon tube whose pressure indication is not adversely influenced by the manner in which attachments are made thereto.

Yet another object of the present invention is to permit the construction of an extremely compact, highly reliable pressure gauge incorporating such a tube.

These and other objects according to the present invention are achieved by the provision of a twisted Bourdon tube for a pressure gauge, which tube is of one piece and is composed of a twisted central portion and two end portions adjoining respective ends of the central portion,

3,463,l l Patented Aug. 26, 1969 each end portion having greater wall thickness than the central portion.

The present invention further includes a pressure gauge composed of a Bourdon tube having the characteristics described above and an angular position indicator having a rotatable member whose angular position determines the indication produced by the indicator, the rotatable member being mounted on one of the end portions of the tube being supported exclusively thereby.

The present invention further includes a method for manufacturing a tube of the type described above from a starting tube made of an initially soft, hardenable material'and having a central portion of noncircular cross section and two end portions each having a greater wall thickness than the central portion. The process according to the present invention includes the steps of rotating the end portions in respectively opposite directions for twisting the central portion about its longitudinal axis, and hardening the twisted tube to render it resilient.

A twisted Bourdon tube having the structure, and produced in the manner, described above present certain substantial advantages.

Firstly, by giving the end portions of the tube a greater wall thickness than the central portion, it is possible to twist only the central portion of the tube during manufacture and to maintain the end portions in an unaltered state. This is very important because if the end portions were also twisted, as for example if the entire tube were cut from a longer twisted tube, the end portions of the resulting tube would deform during operation under the influence of the pressure to be measured and this would cause the properties of the binding material employed for securing the ends of the tube to affect the measurement obtained. Moreover, in such a case the connection to the ends of the tube would tend to become loosened. Since, however, the end portions of the tube according to the present invention do not undergo any torsional deformation, the tube can be mounted and secured in a simple and reliable manner by merely clamping the end portions thereof. Moreover, the manner of attachment of the tube end portions will have no influence whatsoever on the degree of angular deflection of the free end of the tube during use.

Furthermore, it has been found that these characteristics of the tube according to the present invention cause nonlinearities in the tube rotation and variations in the zero adjustment thereof to be reduced to a surprisingly large degree.

Brief description of the drawings FIGURE 1 is a perspective view of a finished twisted Bourdon tube according to the present invention.

FIGURE 2 is a longitudinal cross-sectional view of a Bourdon tube according to the present invention before twisting.

FIGURE 3 is a cross-sectional view taken along the line Ill-III of FIGURE 2.

FIGURE 4 is a cross-sectional view taken along the line IVIV of FIGURE 2.

FIGURE 5 is a longitudinal, cross-sectional view of one embodiment of a complete pressure gauge according to the present invention.

Description of the preferred embodiments The twisted Bourdon tube according to FIGURES l to 4 was manufactured from a continuous tube having a base 7 and initially having the cross section shown in FIGURE 4-. By turning the central portion 3 down to an external radius r, the wall thickness in this portion is reduced, the end portions 1 and 2 retaining their original wall thickness. The transition from the thin-walled central portion to the thick-walled end portions is gradual and has a suitable radius of curvature as shown at 4.

Three longitudinal recesses are then pressed into the central portion, as can be best seen in FIGURE 3, to create a cross section in the form of a three-pointed star having sufficiently large radii of curvature at the tips 5 and the troughs, or fillets, 6 to prevent the formation of cracks while the tube is in use. The recesses gradually taper off at each longitudinal end of the central portion to form a smooth transition with the radius shown at 4. This also contributes to preventing cracks from developing in the tube during subsequent use.

By rotating the two end portions 1 and 2 by approximately 150 with respect to each other, the finished form shown in FIGURE 1 results.

The following procedure may be employed for the manufacture of such a twisted Bourdon tube: The starting material is a tube or elongated hollow body having a base at one end. Over its central portion, the tube, as already mentioned, has thinner walls than do the end portions.

The cross-sectional configuration in the central portion is of primary importance. It must not be circular and must at least be oval. However, it is far preferable for this portion to have star-shaped cross section presenting sufliciently large radii of curvature at the tips and the troughs, or fillets, of the star.

The material for the tube should initially be soft, but should be hardenable, by heat treatment for example, so as to acquire spring-elastic properties. Both end portions of the soft tube, or body, are then clamped into suitable collets and are rotated with respect to each other about the longitudinal axis of the tube. A permanent twist is thus produced only in the area of reduced wall thickness, i.e., only over the central portion. The material is then hardened and made resilient by a subsequent heat treatment.

If one starts out with a smooth tube of uniform, circular cross section, the following steps must precede the above procedure: First the tube wall in the central portion must be given a reduced thickness by turning. The particularly desirable star-shaped cross section is obtained by a radial impression of recesses, which are equally distributed around the tube circumference, which extend in a longitudinal direction and which are only as long as the portion having a reduced wall thickness. No sharp edges are permitted to exist anywhere over the central portion or in the transition regions between the central portion and the end portions. The stamping tools, therefore, must be suitably formed at all surfaces. The ends of the recesses, in particular, are to taper out gradually.

Under certain circumstances it is advantageous to insert a supporting body, e.g., of simple cylindrical section, into the tube so that the stamping tools will act against an abutting surface. In this case, at least the distance between the troughs 6 of the star profile and the tube axis will be positively established.

The supporting body, however, could also be in the form of a cylinder having the cross section of the interior of the desired star profile, the supporting body then serving as a die, or it can additionally possess the form of the entire stamped, but not yet twisted, tube interior, including the end portions thereof. In order to be able to remove the supporting body after stamping, it can be made of several readily separated parts or can be made of a material which is inflammable, thermoplastic, or otherwise destructible.

When one of the latter types of supporting body having a star-shaped cross section is used, no stamping tools of specific shape are required and the soft tube can rather be simply pressed like a skin around the supporting body with the aid of a plastic or rubber-elastic pressure exerting mass and can thus be gently deformed.

The same result can also be achieved by means of a pressurized liquid or gaseous agent in combination with a supporting body having the form of the entire finished tube. In this case, the tube is first provided with a base, which is required in any event, and With the profiled supporting body. The tube is then inserted, base first, into a closed container and has its other, open, end securely connected to the container wall. The container is then filled with the above-mentioned agent, which is placed under pressure. Then the supporting body is removed and the tube is subsequently twisted and hardened.

If the fully supporting body has a twisted shape, i.e., corresponds to the interior of the desired twisted Bourdon tube, the production operation comprising the twisting can be eliminated.

Preferably, however, the tube is first provided with recesses and then twisted. In order to further avoid sharp edges, it is recommended to fill the interior of the tube with fine sand or a viscous mass, such, as, e.g., tin, resin, or the like, during the twisting operation.

The angular deflection sensitivity of the twisted Bourdon tube, i.e., the amount of rotation amount of rotation of its free end as a result of a particular pressure differential, depends on the amount of twist in such a manner that the tighter the twist the smaller the sensitivity. In this connection it should be noted that the tighter the twist, the smaller the angle of inclination of the twists with respect to a radial plane perpendicular to the tube axis. This relation is true at least for inclination angles of between approximately and 84, which range has proven to be suitable for tubes according to the present invention. It has further developed that at an angle of approximately 82 both a distinct maximum deflection sensitivity and optimum linearity are attained.

The twisted Bourdon tube can be made of a conventional resilient materal, such as, for example, copperberyllium consisting of 1.8% to 2.1% B., 0.2% to 0.6% Ni and Co, the remainder being Cu. This material is furnished in a half-hardened state in which it can be machined particularly easly. When the selected piece has been completely machined, it is heated to 720 C. to 760 C. and quenched in Water and thus softened. It can then be stamped and twisted particularly well. Subsequently it is hardened for four hours at 325 C. to give it the desired resiliency.

A twisted Bourdon tube of the type described above can be used to produce a particularly compact pressure gauge having a high degree of reliability Such a gauge preferably includes an inductance type angular position indicator and, in further accordance with the present invention, the rotor of this angular position indicator is mounted at the free end of the twisted Bourdon tube and is supported exclusively thereby. In other words, it is mounted like a floating bearing, to borrow a term used in the machine industry.

One advantage of such a pressure gauge is that neither an intervening mechanical movement of the rotor by means of levers or cog wheels nor a frictional bearing to adversely influence the measuring action is present. When a highly sensitive inductive indicator is used, which as is known, has only a small maximum angular deflection (e.g., 1.5 the twisted Bourdon tube can be made short enough to have a suflicient resistance to radial deflections due to the action of the rotor.

One such pressure gauge is shown in FIGURE 5 to consist of a cylindrical housing 8 having a bottom 9 and a cover 10. The left end of the twisted Bourdon tube 11 is provided with a flange 13 and a threaded ferrule 12, which form a unitary piece with the left end. The Bourdon tube is inserted with its closed right end first into the housing through an opening in bottom 9 and the tube is soldered or welded to the bottom at flange 13. The free right end of tube 11 is inserted into the rotor 14- of a highly sensitive inductive angular position indicator so as to provide the sole support for the rotor. The associated stator 15 with winding 16 is fixedly mounted in housing 8. Two of the visible winding connections 17 are brought to the outside through insulated sleeves 18 in cover 10.

It will be understood that the above description of the present invention is susceptible to various modifications, changes, and adaptations, and the same are intended to be comprehended within the meaning and range of equivalents of the appended claims.

We claim:

1. A twisted Bourdon tube for a pressure gauge, said tube being of one piece and comprising a twisted central portion and two end portions adjoining respective ends of said central portion, each end portion having a greater wall thickness than said central portion.

2. An arrangement as defined in claim 1 wherein each said end portion is of circular cross section.

3. An arrangement as defined in claim 1 wherein said central portion has a star-shaped cross section.

4. An arrangement as defined in claim 1 wherein a gradual transition exists between the wall thickness of said central portion and the wall thicknesses of said end portions.

5. An arrangement as defined in claim 1 further comprising an angular position indicator having a rotatable member whose angular position determines the indication produced by said indicator, said rotatable member being mounted on one of said end portions of said tube and being supported exclusively thereby.

6. A method for manufacturing a twisted Bourdon tube from a starting tube made of an initially soft hardenable material and having a central portion of noncircular cross section and two end portions each having a greater wall thickness than the central portion, said process comprising the steps of: rotating the end portions in respectively opposite directions for twisting the central portion about its longitudinal axis; and hardening the twisted tube to render it resilient.

7. A method as defined in claim 6 comprising the preliminary operation of forming the starting tube from a tube having a central portion and two end portions and having a circular cross section throughout, said operation comprising deflecting the wall of only the central portion inwardly to create longitudinally extending recesses in the central portion.

References Cited UNITED STATES PATENTS 1,315,327 9/1919 Place 73-418 2,877,326 3/1959 Bourns 73418 2,882,503 4/1959 Huff et a1. 73418 3,280,632 10/1966 Harland et a1 73418 CHARLES W. LANHAM, Primary Examiner LOWELL A. LARSON, Assistant Examiner US. Cl. X.R. 72-37l 

